Can you grow directly in compost? Exploring two common growing questions

In the last 12 months we’ve had an increasing number of market gardeners who are looking for alternative inputs to their veggie growing business. Chicken manure has long been the staple but many growers are starting to have issues with soil borne diseases.

Interestingly one of our customers started to grow directly in compost above their existing soil in make shift raised beds which raised the question of whether you can grow directly in compost.

A secondary question we’ve had come up, especially after our newsletter about Reground, is whether you can use spent coffee grinds as a soil amendment.

So this month we report back on the results of some experiments we ran.

Can you grow directly in compost?

One of the markers of quality compost is its maturity. If compost isn’t mature it’s very hard to grow anything directly in it as the decomposition process isn’t complete. In fact immature compost will continue to compost your precious seeds and seedlings, certainly not desirable!

We test maturity using carbon dioxide monitoring and by measuring temperatures. Less CO2 is produced and temperatures cool down once compost begins to mature. Different microbes operate in these conditions and work to break down woody material and give the compost it’s wonderful earthy smell.

Back to our veggie growing customer. Well they kept ordering more compost and converting more of their farm to use compost as the growing medium. This was pretty intriguing, so when I had some new raised beds to fill I tried a 70% compost, 30% coarse sand mix. The coarse sand was recommended by Ash from Southpoint Garden Supplies who had concerns about drainage as compost does such a good job of holding onto moisture – especially at this time of year.

Well so far I’ve been impressed with the results. All plants have been sown from seed including broad beans, carrots, coriander and dill.

So to answer the question – it would seem YES you can grown direct in compost, provided it’s fully mature!

What about spent coffee grinds?

A few years back I was collecting large volumes of coffee grinds from a local cafe as I couldn’t bear to see it end up in landfill.

I’d heard that coffee grinds were acidic so I checked myself with our lab calibrated pH meter. 6.1 – certainly not neutral, but a long way off being acidic. First myth busted.

Gardening and sustainability wisdom everywhere suggests that spent coffee ‘pucks’ are nutritional gold for your garden and should be incorporated into your garden beds to increase the vigour of your veggies.

To test the theory I put a buckets full of this material in the one garden bed and dug it through. I was hoping for big things and put in half a punnet of lettuce seedlings, with the other half going in a neighbouring bed as a control.

What happened next was nothing. Yes, nothing. The lettuce in the coffee bed didn’t die, but didn’t grow. It just froze in time. The control bed seedlings had established and were growing quite well, so decided to dig in the coffee bed. I found worms so that had to be good, right?

Well with a little more research I found the culprit. Coffee is one of the many species of plant which releases allelopathic chemicals which actively discourage competition from other plants. As it turns out, lettuce is one of the most significantly affected species by this form of antisocial phytotoxic behaviour!

Oh the delicious irony…the plant that brings so many of us together actually likes to be by itself. The upshot of all this is it’s always best to compost your coffee before adding it to your garden.